1. Field of Invention
The invention relates generally to the field of non-destructive testing. More specifically, the present invention relates to a system to create a mid-range infrared generation laser beam.
2. Description of Prior Art
Recent developments in creating composite materials have expanded the use of composite materials into a wide variety of applications. Because of its high strength and durability combined with its low weight, composites are replacing metals and metal alloys as the base material for certain load bearing components. For example, composites are now commonly used as a material for body parts and structure in vehicles such as automobiles, watercraft, and aircraft. However, to ensure composite mechanical integrity, strict inspections are required. The inspections are typically required upon fabrication of a component made from a composite and periodically during the life of the component.
Laser ultrasound is one example of a method of inspecting objects made from composite materials. The method involves producing ultrasonic vibrations on a composite surface by radiating a portion of the composite with a pulsed generation laser. A detection laser beam is directed at the vibrating surface and scattered, reflected, and phase modulated by the surface vibrations to produce phase modulated light. Collection optics receives the phase modulated laser light and directs it for processing. Processing is typically performed by an interferometer coupled to the collection optics. Information concerning the composite can be ascertained from the phase modulated light processing, the information includes the detection of cracks, delaminations, porosity, foreign materials (inclusions), disbonds, and fiber information.
FIG. 1 provides one prior art example of a laser system 10 for producing a pulsed generation laser beam. The laser system 10 is configured to produce laser light in the mid infrared range and comprises a mid infrared laser head 11 optically coupled to a mid infrared emission head 30. The mid IR laser head 11 includes a thulium yttrium lithium fluoride (Th:YLF) end pumped by a pair of diode pumps (16, 18). The output 19 of diode pump 18 pumps one end of the Th:YLF laser 20, the other end of the laser 20 is pumped by pump diode 16. Pump diode 16 output beam (not shown) passes through the transmissible side of a dichroic mixer 24 and into the lower end of the laser 20. The thulium laser output 20 is directed towards the reflective side of the dichroic mixer 24 and is reflected towards the emission head 30. Optional input and output couplers (21, 25) are provided on the respective input and output of the thulium laser 20. In this prior art embodiment, the pump diodes (16, 18) pump the thulium laser 20 at a wavelength of 794 nanometers. The thulium output beam 22 operates at approximately 1.94 microns.
The emission head 30 has a holmium yttrium aluminum garnet (Ho:YAG) laser 34 operatively coupled with a frequency converter 38. The frequency converter 38 is depicted as an optical parametric oscillator (OPO). The Ho:YAG laser 34 receives the reflected laser output 26 at a wavelength of approximately 1.94 microns and emits its corresponding output beam 36 at a wavelength of about 2.05 microns. The OPO converts the output beam 36 to a signal beam and an input beam, where the signal beam has a wavelength of about 3.2 microns and the idler beam has a wavelength of about 5.7 microns.
The laser system 10 emits from about five to about ten watts of 3 to 4 micron light, but requires about 1 kilowatt in power of pump diodes. Accordingly, the mid IR laser head 11 is equipped with an associated cooling circuit 14 and power supply 12 that requires a substantial capacity to support laser system 10 operation. The increased power in cooling capacity for the system 10 results in a large volume and a large mass laser head. Additionally, the 794 nanometer pump diodes are not common readily available items. Typically the Ho:YAG laser output is at about 2.05 microns where it is converted within the OPO to a mid IR laser output of about 3.2 microns. A Q-switching device (not shown) is typically included within the emission head 30. Q-switching provides pulsing to the output laser beam 40 for creating the thermo-elastic displacements on a target surface that then forms ultrasonic displacements on the target surface. The Ho:YAG laser 34 is shown with an input coupler 32 at its input and an output coupler 33 at its output. The OPO 38 is illustrated having both an input and output coupler (35, 37).